Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).
An orthogonal FDM (OFDM) technique distributes the data over a large number of carriers that are spaced apart at defined frequencies. This spacing provides the “orthogonality” of the OFDM approach, and prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM include high spectral efficiency, resiliency to RF interference, and lower multipath distortion. This is useful because in a typical terrestrial wireless communications implementation there are multipath channels (i.e., the transmitted signal arrives at the receiver using various paths of different length).
OFDM has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. For example, OFDM has been selected as the multiple access scheme by several standard organizations, including IEEE 802.11, IEEE 802.16, BVB-T (digital video broadcast-terrestrial), and DVB-H (handheld). OFDM beneficially reduces the influence of ISI with a complexity that is less than that of typical single carrier adaptive equalizers. OFDM has also been found to work well in multipath fading channels. These and other advantages render OFDM a strong candidate for use in future mobile communication systems, such as one being referred to as 4G (fourth generation).
While adoption as above in multiple standards proves that OFDM is an excellent candidate for multipath propagation, it is vulnerable to phase noise. Phase noise arises in the up-conversion at the transmitter, and in the down-conversion at the receiver, where the local oscillators behave in less than an ideal manner. An important feature of OFDM is the orthogonality of the sub-carriers. Phase noise from the local oscillators threatens that orthogonality. As would be expected, the sensitivity of any particular OFDM implementation depends from the sub-carrier distance from one another. Low frequency phase noise, typically termed common phase noise CPE, has been corrected in the prior art by rotating the signal constellation with the aid of pilot tones or pilot signals. High frequency phase noise introduces inter-carrier interference ICI. Unlike inter-symbol interference ISI where multiple versions of the same signal interfere with one another due to recovered multipath propagation, ICI appears as additive Gaussian noise to the receiver. Traditionally, this was countered in the prior art by simply using high quality local oscillators to reduce any phase noise imposed from the start. At high frequencies (e.g., about 60 GHz and greater), phase noise imposed even from these higher quality local oscillators does not reduce ICI to a sufficient degree that sub-carrier orthogonality, as seen by the receiver, can be maintained. This is true at least for higher level modulation schemes (e.g., 16 QAM, 64 QAM).
The inventors have devised an approach that corrects phase noise in both low and high frequency regimes, as detailed below, thereby correcting for ICI. While described in the context of OFDM in particular, the phase noise correction techniques described herein do not depend from particularities of OFDM but are readily extendable to any multi-carrier signaling regimen, including multi-carrier CDMA (MC-CDMA) and other multi-carrier communication protocols that may yet be developed that are not inconsistent with these teachings.